Marine controlled source electromagnetic (MCSEM) surveys have become an important part of the offshore petroleum exploration. However, due to enormous computational difficulties of the full 3-D inversion, the practical interpretation of the MCSEM data is still based on qualitative analysis and relatively simple 2-D geoelectrical models of the sea-bottom structures. In this paper we present a new approach to 3-D inversion of the MCSEM data. It is based on the rigorous integral equation (IE) forward modeling and a new IE representation of the sensitivity (Frechet derivative matrix) of the observed data to the variations of the sea-bottom conductivity. This approach requires just one forward modeling on every iteration of the regularized gradient type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method, which provides a sharp boundary image of the petroleum reservoir. The methodology is tested on a 3-D inversion of the synthetic EM data representing a typical MCSEM survey conducted for offshore petroleum exploration.